1. Field of the Invention
This invention relates to a process for reducing the emission of sulfur oxides from the regenerator of a catalytic cracking unit. More particularly, the invention relates to the use of regenerable sulfur oxide absorbents which are circulated through the catalytic cracking process in combination with the cracking catalyst.
2. Description of the Prior Art
A major industrial problem involves the development of efficient methods for reducing the concentration of air pollutants, such as sulfur oxides, in waste gas streams which result from the processing and combustion of carbonaceous fuels which contain sulfur. The discharge of these waste gas streams into the atmosphere is environmentally undesirable at the sulfur oxide concentrations which are frequently encountered in conventional operations. The regeneration of cracking catalyst which has been deactivated by coke deposits in the catalytic cracking of sulfur-containing hydrocarbon feedstocks is a typical example of a process which can result in a waste gas stream containing relatively high levels of sulfur oxides.
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
In the catalytic cracking of hydrocarbons, some nonvolatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons and generally contains from about 4 to about 10 percent hydrogen. When the hydrocarbon feedstock contains organic sulfur compounds, the coke also contains sulfur. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stocks diminishes.
Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. The catalyst particles are then reactivated to essentially their original capabilities by burning the coke deposits from the catalyst surfaces with an oxygen-containing gas such as air in a regeneration zone. Regenerated catalyst is continuously returned to the reaction zone to repeat the cycle.
When sulfur-containing feedstocks, such as petroleum hydrocarbons containing organic sulfur compounds, are utilized in a catalytic cracking process, the coke deposited on the catalyst contains sulfur. During regeneration of the coked deactivated catalyst, the coke is burned from the catalyst surfaces which results in the conversion of the sulfur to sulfur dioxide together with small amounts of sulfur trioxide. This burning can be represented, in a simplified manner, as the oxidation of sulfur according to the following equations: EQU S (in coke)+O.sub.2 .fwdarw.SO.sub.2 ( 1) EQU 2SO.sub.2 +O.sub.2 .fwdarw.2SO.sub.3 ( 2)
One approach to the removal of sulfur oxides from the waste gas produced during the regeneration of deactivated cracking catalyst involves scrubbing the gas downstream of the regenerator vessel with an inexpensive alkaline material, such as lime or limestone, which reacts chemically with the sulfur oxides to give a nonvolatile product which is discarded. Unfortunately, this approach requires a large and continual supply of alkaline scrubbing material, and the resulting reaction products can create a solid waste disposal problem of substantial magnitude. In addition, this approach requires complex and expensive auxiliary equipment.
A second approach to the control of sulfur oxide emissions involves the use of sulfur oxide absorbents which can be regenerated either thermally or chemically. An example of this approach to the removal of sulfur oxides from the regeneration zone effluent gas stream in a cyclic, fluidized catalytic cracking process is set forth in U.S. Pat. No. 3,835,031 to Bertolacini et al. This patent discloses the use of a zeolite-type cracking catalyst which is modified by impregnation with one or more metal compounds of Group IIA of the Periodic Table, followed by calcination, to provide from about 0.25 to about 5.0 weight percent of Group IIA metal or metals as an oxide or oxides. The metal oxide or oxides react with sulfur oxides in the regeneration zone to form non-volatile inorganic sulfur compounds. These non-volatile inorganic sulfur compounds are then converted to the metal oxide or oxides and hydrogen sulfide upon exposure to hydrocarbons and steam in the reaction and steam stripping zones of the process unit. The resulting hydrogen sulfide is disposed of in equipment which is conventionally associated with a fluidized catalytic cracking process unit. Belgian Pat. No. 849,637 is also directed to a process wherein a Group IIA metal or metals are circulated through a cyclic fluidized catalytic cracking process with the cracking catalyst in order to reduce the sulfur oxide emissions resulting from regeneration of deactivated catalyst.
U.S. Pat. No. 4,153,534 to Vasalos discloses a process similar to that set forth in U.S. Pat. No. 3,835,031, which involves the removal of sulfur oxides from the regeneration zone flue gas of a cyclic, fluidized catalytic cracking unit through the use of a zeolite-type cracking catalyst in combination with a regenerable sulfur oxide absorbent which absorbs sulfur oxides in the regeneration zone and releases the absorbed sulfur oxides as a sulfur-containing gas in the reaction and steam stripping zones of the process unit. The sulfur oxide absorbent comprises at least one free or combined element selected from the group consisting of sodium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, the rare earth metals and lead.
U.S. Pat. No. 4,071,436 to Blanton et al. teaches that alumina and/or magnesia can be used to absorb sulfur oxides from a gas at a temperature in the range from 1000.degree. to 1500.degree. F. and the absorbed sulfur oxides can be removed by treatment with a hydrocarbon at a temperature in the range from 800.degree. to 1300.degree. F. It is further disclosed that sulfur oxide emissions from the regeneration zone of a cyclic, fluidized catalytic cracking unit can be reduced by combining alumina and/or magnesia with the hydrocarbon cracking catalyst. Similarly, U.S. Pat. No. 4,115,249 to Blanton et al. teaches that a cracking catalyst can be impregnated with an aluminum compound and utilized in a cyclic, fluidized catalytic cracking process for the purpose of reducing regenerator sulfur oxide emissions. Further, U.S. Pat. No. 4,166,787 to Blanton et al. discloses that a finely divided particulate alumina can be physically incorporated into the cracking catalyst for the purpose of reducing regenerator sulfur oxide emissions.
U.S. Pat. No. 4,153,535 to Vasalos et al. is directed to a process of the type set forth in U.S. Pat. No. 4,153,534, and teaches that an improved control of regeneration zone sulfur oxide emissions can be achieved by combining a sulfur oxide absorbent with a metallic promoter. The metallic promoter comprises at least one free or combined element selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, platinum, vanadium, tungsten, uranium, zirconium, rhenium and silver. The sulfur oxide absorbent comprises at least one free or combined element which is selected from the group consisting of sodium, magnesium, calcium, strontium, barium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, lead and the rare earth metals. Similarly, U.S. Pat. No. 4,146,463 to Radford et al. discloses a process wherein a separately generated waste gas containing sulfur oxides and/or carbon monoxide is conveyed to the regeneration zone of a cyclic, fluidized catalytic cracking unit where these pollutants are removed by contact with a sulfur oxide absorbent and, if desired, an oxidation promoter, wherein the absorbent is a metal oxide which reacts with the sulfur oxides to form nonvolatile inorganic sulfur compounds and the promoter comprises at least one free or combined metallic element selected from Groups IB, IIB and III-VIII of the Periodic Table.
An article entitled "Bench-Scale Investigation On Removing Sulfur Dioxide from Flue Gases," by Bienstock et al. in Journal of the Air Pollution Control Association, Vol. 10, No. 2, April 1960, pp. 121-125, discloses the results of a screening program which involved the preparation and testing of a variety of common metallic oxides as sulfur dioxide absorbents. It is further disclosed that reducing gases such as producer gas, hydrogen, and carbon monoxide can be used to regenerate a spent alkalized alumina absorbent. Similarly, an article entitled "Removal of SO.sub.2 from Waste Gases by Reaction with MnO.sub.x on Gamma-Alumina," by Van den Bosch (Proceedings of the 3rd International Symposium On Chemical Reaction Engineering; International Symposium On Chemical Reaction Engineering 3rd, Northwestern University, 1974; Volume I, Chemical Reaction Engineering-II, pp. 571-587) suggests the use of MnO.sub.x on gamma-alumina in a cyclic regenerative operation consisting of reaction with the sulfur dioxide in a flue gas and regeneration of the spent absorbent with hydrogen at elevated temperatures. Further, an article entitled "Reduction of Sulfates by Hydrogen," by Habashi et al. in the Canadian Journal of Chemistry, Vol. 54, 1976, pp. 3646-3650, discloses the results of an experimental evaluation of the effect of hydrogen on a plethora of metal sulfates at elevated temperature. However, none of these three references contains any mention of a catalytic cracking process, and they all fail to suggest any method for reducing the production of sulfur oxides in the regenerator of a fluid catalytic cracking unit.